Balding (4,2,3)
I believe the answer is:
thin on top
'balding' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for thin on top that I've seen before include "Balding (colloq.)" , "Result of losing hair, balding" , "not so hairy now" , "developing a shine for the Head?" , "with less than full head" .)